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Structure of the Galaxy

Passing around the sky there is a broad region that is readily seen to be brighter than the rest of the night sky. It has been traced from the summer constellation Sagittarius northward through Cyngus into Perseus, then southward to Orion (winter sky) into Centaurus (Southern Hemisphere sky) then back northward into Sagittarius. Even a small telescope or pair of binoculars reveals this band to be bright because of the cumulative effect of millions of faint stars. This is the Milky Way. That it is due to myriads of faint stars distributed in a great circle about the position of the Sun shows the Galaxy's basic structure, the manner in which the stars and interstellar material that make up the Galaxy are distributed in space, is flat. This is the plane of the Galaxy, where the greater part of the stars and interstellar material exist. The brightest part of Milky Way, visible low on the southern horizon in the summer sky toward the constellation of Sagittarius, is bright because the star density increases in this direction. This is the direction to the center of the Galaxy, though starlight coming from the vast bulk of stars in this direction is invisible because of the absorption by the dust.

The distribution of dusty, absorption nebulae is very patchy, and there are “windows,” directions passing close to the center in which there is relatively little absorption, that allow study of the distant stars. In these directions and elsewhere in the halo of the Galaxy, the distribution of RR Lyrae and other stars yields its density structure. In the same manner, the directions and distances to the globular clusters may be mapped in three dimensions. The clusters are concentrated in the direction of Sagittarius, and their density decreases outward, allowing astronomers to outline the outer structure of the Galaxy. From their distribution, the position of the densest part of the Galaxy, the center, may be determined. The galactocentric distance of the Sun is currently estimated as R ⊙ ≈ 8 Kpc (25,000 ly).

The brightest stars at the center of the Galaxy may also be studied using long wavelength infrared radiation. The total extent of the plane of the Galaxy can be deduced by analyzing observations of the 21‐centimeter radiation of neutral hydrogen 360° around the plane. This analysis gives the size of the whole Galaxy as about 30,000 pc diameter (100,000 ly). Scans in 21‐cm above and below the plane, together with observations of stars perpendicular to the plane, give a total thickness of about 500 pc (1,600 ly), with half the gas mass within 110 pc (360 ly) of the center of the plane. Radio studies also reveal that the fundamental plane of the Galaxy is warped, like a fedora hat, with the brim pushed up on one side and down on the other (see Figure 1.)

Figure 1

An external view of the Milky Way, looking edge‐on or sideways into the disk.

It is bent down on Sun side of the Galaxy and up on the opposite side, due to a gravitational resonance with the Magellanic Clouds, which move in an orbit about the Milky Way.

While the greater part of the mass of the Milky Way lies in the relatively thin, circularly symmetric plane or disk, there are three other recognized components of the Galaxy, each marked by distinct patterns of spatial distribution, motions, and stellar types. These are the halo, nucleus, and corona.

Disk

The disk consists of those stars distributed in the thin, rotating, circularly symmetric plane that has an approximate diameter of 30,000 pc (100,000 ly) and a thickness of about 400 to 500 pc (1,300 to 1,600 ly). Most disk stars are relatively old, although the disk is also the site of present star formation as evidenced by the young open clusters and associations. The estimated present conversion rate of interstellar material to new stars is only about 1 solar mass per year. The Sun is a disk star about 8 kpc (25,000 ly) from center. All these stars, old to young, are fairly homogeneous in their chemical composition, which is similar to that of the Sun.

The disk also contains essentially all the Galaxy's content of interstellar material, but the gas and dust are concentrated to a much thinner thickness than the stars; half the interstellar material is within about 25 pc (80 ly) of the central plane. Within the interstellar material, denser regions contract to form new stars. In the local region of the disk, the position of young O and B stars, young open clusters, young Cepheid variables, and HII regions associated with recent star formation reveal that star formation does not occur randomly in the plane but in a spiral pattern analogous to the spiral arms found in other disk galaxies.

The disk of the Galaxy is in dynamical equilibrium, with the inward pull of gravity balanced by motion in circular orbits. The disk is fairly rapidly rotating with a uniform velocity about 220 km. Over most of the radial extent of the disk, this circular velocity is reasonably independent of the distance outward from the center of the Galaxy.

Halo and bulge

Some stars and star clusters (globular clusters) form the halo component of the Galaxy. They surround and interpenetrate the disk, and are thinly distributed in a more or less spherical (or spheroidal) shape symmetrically around the center of the Milky Way. The halo is traced out to about 100,000 pc (325,000 ly), but there is no sharp edge to the Galaxy; the density of stars simply fades away until they are no longer detectable. The halo's greatest concentration is at its center, where the cumulative light of its stars becomes comparable to that of the disk stars. This region is called the (nuclear) bulge of the Galaxy; its spatial distribution is somewhat more flattened than the whole halo. There is also evidence that the stars in the bulge have slightly greater abundances of heavy elements than stars at greater distances from the center of the Galaxy.

The halo stars consist of old, faint, red main sequence stars or old, red giant stars, considered to be among the first stars to have formed in the Galaxy. Their distribution in space and their extremely elongated orbits around the center of the Galaxy suggest that they were formed during one of the Galaxy's initial collapse phases. Forming before there had been significant thermonuclear processing of materials in the cores of stars, these stars came from interstellar matter with few heavy elements. As a result, they are metal poor. At the time of their formation, conditions also supported the formation of star clusters that had about 10 6 solar masses of material, the globular clusters. Today there exists no interstellar medium of any consequence in the halo and hence no current star formation there. The lack of dust in the halo means that this part of the Galaxy is transparent, making observation of the rest of the universe possible.

Halo stars can easily be discovered by proper motion studies. In extreme cases, these stars have motions nearly radial to the center of the Galaxy—hence at right angles to the circular motion of the Sun. Their net relative motion to the Sun therefore is large, and they are discovered as high‐velocity stars, although their true space velocities are not necessarily great. Detailed study of the motions of distant halo stars and the globular clusters shows that the net rotation of the halo is small. Random motions of the halo stars prevent the halo from collapsing under the effect of the gravity of the whole Galaxy.

Nucleus

The nucleus is considered to be a distinct component of the Galaxy. It is not only the central region of the Galaxy where the densest distribution of stars (about 50,000 stars per cubic parsec compared to about 1 star per cubic parsec in the vicinity of the Sun) of both the halo and disk occurs, but it is also the site of violent and energetic activity. The very center of the Galaxy harbors objects or phenomena that are not found elsewhere in the Galaxy. This is evidenced by a high flux of infrared, radio, and extremely short wavelength gamma radiation coming from the center, a specific infrared source known as Sagittarius A. Infrared emissions in this region show that a high density of cooler stars exists there, in excess of what would be expected from extrapolating the normal distribution of halo and disk stars to the center.

The nucleus is also exceptionally bright in radio radiation produced by the interaction of high‐velocity charged particles with a weak magnetic field ( synchrotron radiation). Of greater significance is the variable emission of gamma rays, particularly at an energy of 0.5 MeV. This gamma‐ray emission line has only one source—the mutual annihilation of electrons with anti‐electrons, or positrons, the source of which in the center has yet to be identified. Theoretical attempts to explain these phenomena suggest a total mass involved of 10 6–10 7 solar masses in a region perhaps a few parsecs in diameter. This could be in the form of a single object, a massive black hole; similar massive objects appear to exist in the centers of other galaxies that show energetic nuclei. By the standards of such active galaxies, however, the nucleus of the Milky Way is a quiet place, although interpretations of the observed radiation suggest the existence of huge clouds of warm dust, rings of molecular gas, and other complex features.

Exterior to the halo

The gravitational influence of the Galaxy extends to an even greater distance of about 500,000 pc (1,650,000 ly) (the late astronomer Bart Bok suggested this region could be called the corona of the Galaxy). In this volume there appears to be an excess of dwarf galaxies associated with the Milky Way, drawn into its proximity by its large gravitational pull. This includes the Magellanic Clouds, which lie in the debris of the Magellanic Stream. The Magellanic Stream consists of a band of hydrogen gas and other materials that extends around the Galaxy, marking the orbital path of these companion galaxies. The tidal gravitational field of the Galaxy apparently is ripping them apart, a process that will be completed in the next two to three billion years. This galactic cannibalism, the destruction of small galaxies, and the accretion of their stars and gas into a larger galactic object likely has happened in the past, perhaps many times. A second, small companion galaxy in the direction of Sagittarius (the Sagittarius galaxy) appears to be another victim of this process. Like the Magellanic Clouds, its stars and interstellar material will ultimately be incorporated into the body of the Milky Way. The total number of dwarf galaxies near the Milky Way is about a dozen and includes objects such as Leo I, Leo II, and Ursa Major. A similar cloud of dwarf galaxies exists about the Andromeda Galaxy.

Rotation curve of the Galaxy

An alternative means of studying the structure of the Galaxy, complementary to looking at the distribution of specific objects, is to deduce the total distribution of mass. This may be done by analyzing the rotation curve, or the circular velocity V(R) of the disk objects moving around the center of the Galaxy as a function of the distance R out from the center. A check on the accuracy of the deduced motion in the Galaxy is given by the rotation curves of similar galaxies, which would be expected to rotate in the same basic fashion. Like the Milky Way, the rotations of other galaxies show a linear increase of velocity near their centers rising to a maximum value and then becoming basically constant over the remainder of the disk.

Determination of V(R) from within the Galaxy is not as straightforward as measuring the rotation of another galaxy that is observed from outside. Observation of neighboring stars or of interstellar gas gives only relative motions. Thus, calculating the absolute solar velocity involves first looking at nearby galaxies and determining what direction the Sun appears to be moving in.

The Sun and its neighboring stars are found to be moving about the center of the Galaxy with a speed of 220 km/s in the direction of the northern constellation Cygnus, at a right angle to the direction towards the center. In the galactic coordinate system used by astronomers, this movement is toward a galactic longitude of 90°. Sweeping around the Galaxy in its plane, galactic longitude starts at 0° toward the center, increases to 90° in the direction of rotation (Cygnus), to 180° in the anti‐center direction (Orion), to 270° in the direction from which the Sun moves (Centaurus), and finally to 360° when the direction of the center is again reached. Use of Doppler shifts and proper motions applied to stars near the sun provide some idea of the local rotation curve; nearby disk stars on average appear to move in circular orbits about the center with the same circular velocity as the Sun. The interstellar dust prevents study by optical techniques of the rest of the Galaxy; thus, the 21‐centimeter radiation of neutral hydrogen must be used to determine its pattern of motion. Again, the Doppler Shift gives only a relative or line‐of‐sight velocity for the gas anywhere in the Galaxy, but knowledge of the solar velocity and geometry allows calculation of the velocity at other radii from the galactic center.

The rotation curve of the Galaxy shows that it does not rotate as a solid disk (velocity directly proportional to distance out from the rotational axis). Rather, the rotational velocity is more or less constant over most of the disk (see Figure 2).

Figure 2

Rotation curve of the Galaxy. If the greatest portion of the mass of the Galaxy were concentrated at its center, then orbital motions would rapidly decrease with radius (dashed line) in the manner of the planetary motions about the Sun described by Kepler.

Viewed as a giant race course, this means that on average all stars move the same distance in a given amount of time, but because the circular paths of outer stars are larger than those closer to the center, the outer stars slip progressively behind the inner stars. This effect is called differential rotation, and it has significant effects on the distribution of star‐forming regions; any large star‐forming region will be sheared into a spiral arc. If the Galaxy rotated as a solid disk, there would be no differential rotation.

Stars, including the Sun, have small components of motion that deviate from a pure circular motion about the center of the Galaxy. This peculiar motion for the Sun is about 20 km/s, a small drift in the general direction of the bright summer star Vega. This results in an approximate 600 pc (1900 ly) in‐and‐out deviation from a true circular orbit as the Sun orbits the center of the Galaxy with a period of 225 million years. A second consequence is an oscillation, with a much shorter period of about 60 million years, up and down through the plane of the disk. In other words, the Sun moves up and down about four times during each trip around the center of the Galaxy. This oscillation has an amplitude of 75 pc (250 ly). At present, the Sun is 4 pc (13 ly) above the galactic plane, moving upward into the Galaxy's Northern Hemisphere.

Mass distribution

In one sense, the Galaxy is analogous to the solar system: The flatness is the result of the operation of the same physical laws. As the material of both contracted at their time of formation, conservation of angular momentum resulted in increased rotational velocities until a balance against gravity was achieved in an equatorial plane. Material above or below that plane continued to fall inwards until the mass distribution became flat. In specific detail, the mass distributions are very dissimilar. The mass of the Galaxy is distributed through a large volume of space, whereas the mass of the solar system is essentially only that of the Sun and is located at the center. The flat disk of the Galaxy implies that rotation plays the dominant role in the balance against gravitation, which, in turn, depends on the mass distribution. The mass M(R) as a function of radius R is determined by applying a modification of Kepler's Third Law to the rotation curve V(R), to obtain

where G is the gravitational constant. Thus, astronomers can determine the mass structure of the Galaxy. Its total mass may be as great as 10 12 solar masses.

Because the mass in the Galaxy is distributed over a large volume, the pattern of rotation differs from that in the solar system. For the planets, orbital velocities decrease with radial distance outward, V(R) ∝ R ‐1/2 (Keplerian motion); in the Galaxy, the circular velocity rises linearly V(R) ∝ R near the center, and then is relatively unchanging over the remainder of the disk, V(R) ∝ constant. This form of rotation curve implies a relatively constant mass density near the center; but further out, the density decreases inversely with the square of the radius.

The motions of the stars are also affected by the spatial distribution of the mass. The nature of Newtonian gravity is that a circularly or spherically symmetric mass distribution always exerts a force toward the center, but this force depends only on that part of the mass that is closer to the center than the object that feels the force. If a star moves outward in the Galaxy, it feels the gravitational force from a larger fraction of the total mass; when it moves closer to the center, less of the mass is exerting a force on the object. As a result, orbits of stars are not closed ellipses like those of the planets, but instead more closely resemble the patterns produced by a spirograph. Additionally, a planetary orbit is a flat plane; hence, if that orbit is inclined to the overall plane of the solar system, in one complete circuit about the Sun the planet moves once above and once below the solar system plane. A star, however, will oscillate up and down several times in one passage around the center of the Galaxy.

Spiral arm phenomenon

In the Galaxy, the mass structure of the disk is not perfectly smooth. Instead, there are regions in the disk where the density of stars is slightly larger than the average. In these same regions, the density of the interstellar material may be significantly larger. These density variations, or fluctuations, are not completely random; they show a global pattern of spirality, or spiral arms, within the disk (see Figure 3). Again the dust in our Galaxy is a problem; thus, spiral features easily studied in distant disk galaxies can give us insight to the pattern in the Milky Way. Stellar and nonstellar objects associated with the spiral arms can be mapped out only locally in our Galaxy, out to 3 kpc (10,000 ly) or so, because in regions of higher density of interstellar material, star formation occurs. In particular, the brightest O and B stars are indicative of the most recent star formation. They and other objects associated with recent star formation (emission regions, Cepheid variables, young star clusters) may be used as optical tracers of the spiral arm pattern. Analysis of 21‐centimeter observations is more difficult, but suggests that coincident with young stellar objects are the denser regions of interstellar material.

Figure 3

A schematic interpretation of the spiral features in the disk of the Milky Way Galaxy. The various spiral arms are named after the constellations in which directions their brightest features are observed.

To have a pattern of compression (higher density) and rarefaction (lower density) in the spiral arm pattern that exists over the whole disk of a galaxy requires energy, in the same manner that the sound produced when a person speaks requires energy. Both phenomena are examples of wave phenomena. A sound wave is a pattern of alternate compression and rarefaction in air molecules. Like any wave phenomena, the energy that is responsible for the wave will dissipate into random motions, and the wave pattern should die away in a relatively short period of time.

The density wave that passes through the disk of the Galaxy can be better related to the density waves that are found on freeways. At times, any given driver will be in the midst of “traffic,” but at other times, he or she will seem to be the only driver on the road. Physically, these waves are the result of two factors. First, not all automobiles are driven at the same speed. There are slower and faster drivers. Second, congestion occurs because there are a limited number of lanes for the traffic flow. Faster drivers come up from behind and are delayed as they weave from lane to lane in their effort to get through to the head of the pack and resume their higher speed. They then can rush ahead, only to get caught up in the next pattern of congestion. Slower drivers get left behind until the next traffic wave catches up to them. Seen from a helicopter, a wave of alternatively denser and thinner distributions of cars is traveling down the highway; those cars in the dense regions, however, change as the faster cars move through and the slower ones drift behind.

In the Galaxy, the dynamics are slightly different in that the “highway” is a circulation about a galactic center, and the congestion is due to the stronger gravity in regions with larger numbers of stars. The spiral density wave theory begins by postulating the existence of a spirally structured pattern of density enhancement in a galactic disk. In the regions of extra density, the extra gravity affects motions and causes the gas and stars to “pile up” momentarily in these spirally shaped regions. Once the stars have passed through the spiral arm, they can move slightly faster until they catch up to the next spiral arm where they again will be momentarily delayed. The gas particles, being much less massive than the stars, are significantly more affected by the excess gravity and can be compressed to five times the average density of the interstellar matter in the disk. This compression is enough to trigger star formation; the newly formed luminosity O and B stars and their associated emission regions thus light up the regions of the spiral arms. The theory very successfully shows that a spiral density enhancement in the form of two well‐formed spiral arms, a so‐called Grand Design, is self‐sustaining for several rotations of a galaxy. In the Milky Way, the expected flow pattern in stellar motions due to acceleration by the gravity of the spiral arms, superimposed on the overall circular motion about the center of the Galaxy, has been observed.

The evidence for the excitation of the wave in the first place should be evident because the lifetime of such a wave is rather short (a few galaxy rotation periods). In fact, a Grand Design spiral galaxy is generally accompanied by a companion galaxy whose recent close passage by the larger galaxy gave the gravitational stimulus to produce the density wave.

Not all galaxies show a distinct, two‐armed spiral pattern. In fact, the majority of disk galaxies show numerous arc‐like features, apparent fragments of spiral features that are referred to as flocculent galaxies. Each arc represents a region lit up by the bright stars of recent star formation and are explained by the stochastic self‐propagating star formation theory. Given an initial collapse of interstellar gas into a group of stars, in due course a massive star will undergo a supernova explosion. Shock waves moving outward then push the ambient interstellar material into denser condensations and can trigger a next generation of new stars. If there are new massive stars, there will be subsequent supernovae, and the process repeats (the self‐propagating aspect). This cycle continues until the interstellar gas is depleted, or until by chance no new massive stars form (this is the random, or stochastic, aspect of this theory). During the existence of a wave of star formation moving outwards from some original position, however, the growing region of star formation is affected by differential rotation in the disk; the outer part of the star‐forming region lags behind the inner part. The region of star formation is therefore smeared into a spiral arc, as would be all other growing, star‐forming regions elsewhere in the disk; but there would be no grand design.